Vehicle door proximity detector system

ABSTRACT

A proximity detector for use on vehicle doors provides a pair of parallel connected branch circuits each providing an antenna mounted along the leading edge of a vehicle door and a serially connected input winding of a transformer with both antennae providing a capacitive coupling to the electrical ground of the elevator structure. The pair of transformers each provide an output winding which are serially connected in electrical opposition and provide an output in response to a change in impedance to ground of one antenna with respect to the other. The pair of branch circuits are connected to a source of periodic voltage and are further connected to a shunting circuit having a variable impedance which electrically shunts each antenna to the electrical ground through variable resistive and capacitive type elements. A variable capacitor is parallel connected across one transformer input winding while a control circuit is connected to the output windings and includes an interconnected operational amplifier, an AC to DC conversion circuit, an electrical comparator circuit providing a dis-connect and trigger-level control, and a switching circuit providing a controlled rectifier and an output relay.

United States Patent [19] Cohen [451 Nov. 5, 1974 1 VEHICLE DOOR PROXIMITY DETECTOR SYSTEM [75] Inventor: David J. Cohen, Skokie, 111.

[73] Assignee: A. O. Smith Corporation,

Milwaukee, Wis,

[22 Filed: June4,1973

211 Appl.No.:366,754

Primary ExaminerRobert K. Schaefer,

Assistant Examiner-W. E. Duncanson, Jr.

Attorney, Agent, or F irm--Andrus, Sceales, Starke & Sawall [57] ABSTRACT A proximity detector for use on vehicle doors provides a pair of parallel connected branch circuits each providing an antenna mounted along the leading edge of a vehicle door and a serially connected input winding of a transformer with both antennae providing at capacitive coupling to the electrical ground of the elevator structure. The pair of transformers each provide an output winding which areserially connected in electrical opposition and provide an output in response to a change in impedance to ground of one antenna with respect to the other. The pair of branch circuits are connected to a source of periodic voltage and are further connected to a shunting circuit having a variable impedance which electrically shunts each antenna to the electrical ground through variable resistive and capacitive type elements. A variable capacitor is parallel connected across one transformer input winding while a control circuit is connected to the output windings and includes an interconnected operational amplifier, an AC to DC conversion circuit, an electrical comparator circuit providing a dis-connect and trigger-level control; and a switching circuit providing a controlled rectifier and an output relay.

12 Claims, 3 Drawing Figures VEHICLE DOOR PROXIMITY DETECTOR SYSTEM BACKGROUND OF THE INVENTION This invention relates to. a safety control system utilizing proximity detection for power operated vehicle doors which may be advantageously used with elevator systems which are automatically operated without operators or attendants although the invention is not so limited.

Safety devices have been employed with elevator installations in which elevator cars are operated without attendants to protect the various loads carried by the cars against injury by the closing doors. Several types of systems employ one or more antennae mounted along the leading edge of the elevator car door which detect the presence of objects or loads within a certain distance of the door. Such systems operate by detecting a change in electrical capacity of the antennae to an electrical ground established by the structure caused by the proximity of the sensed object or load. The system thus customarily operates to prevent the further closing of the door and possibly to reopen the door in response to the sensed presence of an object within the closing path thereof.

Some of the previous proximity detection systems have employed electronic gaseous discharge tubes which are connected to each antenna and are fired or rendered in a condition to conduct when detecting the presence of an object. Other systems have employed one or more electrical bridge circuits with an oscillating source connected across one bridge diagonal while sensing an output across the other diagonal in response to an unbalance of impedance existing between any two bridge arms, i.e., the two bridge antennae.

SUMMARY OF THE INVENTION This invention relates to a proximity detector system transformer means provides an input winding means which is connected in circuit to the capacitance means to form a pair of parallel connected electrical paths. The pair of electrical paths each include a different portionof the capacitance means and a different portion of the transformer means and are energized by a source of periodic voltage. The transformer means further includes output winding means coupled to the different portions of the input winding means for establishing a pair of oppositely polarized signals in accordance with the current flow in the pair of parallel paths. A control circuit is connected to the output winding means and to a door operating means'to selectively modify the operation of the door.

In a preferred form of the invention, the capacitance means includes a pair of antennae which are each serially connected to an input winding of a pair of output transformers thereby providing a pair of parallel connected branch circuits each of which is coupled to an electrical ground provided by the vehicle structure through the capacitive coupling between the antennae and the structure. The pair of output transformers further provide output windings which are connected in series electrical opposition thereby providing an output 2 in response to a change in impedance to ground of one antenna with respect to the other due to the proximity of load to such antenna.

In one aspect of the invention, a variable impedance means is connected to the capacitance means and to the source and includes a common adjustable output means to provide an adjustable, inversely proportional impedance in the parallel paths and in parallel with the different portions of the capacitance means.

In a preferred form of the invention, the variable impedance provides a shunting circuitwhich is connected to the electrical ground and hence to the source and to the junctures between the antennae and the input windings thereby providing a shunting impedance between each antenna and the electrical ground. Specifically, several resistive type elements are serially connected between the two defined junctures with one resistive element providing a selectively variable output tap which iselectrically coupled to the electrical ground through several capacitive type elements including a variable capacitor.

In another aspect of the invention, a selectively variable capacitive element is electrically connected in par- An electrical comparator circuit provides a first input which is electrically connected to receive the output from the conversion circuit. A second input of the electrical comparator is connected to a preset voltage which is utilized to establish a triggering level for the operation of the comparator. In operation, the varying magnitude of the DC. output voltage provided by the conversion circuit must reach the present voltage level .supplied at the second input in order for the comparator to provide an output control signal for modifying the operation of the vehicle door.

A switching circuit is connected to receive the output signal from the comparator and is electrically coupled to supply a control signal to the door operating means for modifying the operation of the door. In a preferred embodiment, the switching circuit includes a controlled rectifier having a gating input connected to the comparator output and an anode which is connected to a voltage source through a control relay.

Another important aspect of the invention includes a de-activating circuit which is selectively electrically connected to the first input of the comparator circuit in response to the door attaining a preselected position to prevent the switching circuit from modifying the operation of the door. In a preferred embodiment of the invention, a door close limit switch provides contacts electrically connected in circuit with the first input of the comparator and operates in response to the substantial closure of the door to complete an electrical circuit from the first input to the circuit ground for pre venting the comparator from providing an output signal to modify the door operation.

geously be used in vehicular systems such as elevators which is highly sensitive to loads sensed within the closing path of the vehicle door. The system is readily adjustable to provide continued stability of the sensing and control circuitry when used under normal operating conditions.

BRIEF DESCRIPTION OF THE DRAWING The drawing furnished herewith illustrates the best mode presently contemplated by the inventor and clearly discloses the above advantages and features as well as others which will be readily understood from the detailed description thereof:

In the drawing:

FIG. 1 is a diagrammatic view illustrating the preferred installation of the proximity detector to an elevator car spaced from the hoistway door;

FIG. 2 is a section taken at the line 22 in FIG. 1 illustrating the mounting of the pair of antennae within the proximity detector; and

FIG. 3 is a circuit schematic illustrating the electrical circuit of the proximity detector.

DESCRIPTION OF THE PREFERRED ILLUSTRATED EMBODIMENT The invention as illustrated in the drawing is utilized with an elevator system in which a car moves through a building hoistway to transport loads such as passengers for serving a plurality of landings. The disclosed invention is particularly applicable to elevator systems in which the cars are operated without attendants and the car and hoistway doors are automatically operated by power-operated door operating means in response to commands dictated by a supervisory control system. Such supervisory controlled door control circuits and systems are well known to those skilled in the art and a detailed description thereof is deemed unnecessary to a full and clear understanding of the subject invention.

FIG. 1 diagrammatically illustrates the connection of a proximity detector I to the leading edge of the car door 2 and spaced from the hoistway door 3. FIG. 2 diagrammatically illustrates the proximate positioning of the sensing elements within the detector 1 which includes a pair of vertically spaced antennae 4 and 5 placed in line near the leading edge of the car door, each of which consists of a flat metallic plate. The detector 1 generally includes a metallic housing 6 which encloses the antennae 4 and 5 on three sides so that one side of each antenna faces the leading edge of the door. An insulating material 7, such as a high impact plastic, completely encloses the metallic housing 6 and the antennae 4 and 5. During operation, the detector housing 6 is customarily energized by the electrical system reference potential at 17 while the insulator 7 provides protection to the passengers and other loads.

The proximity detector circuit is diagrammatically illustrated in the schematic set forth in FIG. 3. Specifically, the pair of antennae 4 and 5 each form essentially a capacitive type electrical element by the capacitive coupling thereof to an electrical ground 8 generally provided by the structure or building and designated as Cgl and Cg2. The antennae 4 and 5 are each connected in a branch circuit 9 and 10, respectively, and are serially connected to transformers 11 and 12, re-

spectively. Specifically, antenna 4 is serially connected to an input winding 13 of transformer 11 forming a first electrical path while antenna 5 is serially connected to an input winding 14 of transformer 12 forming a second electrical path. The branch circuit 9 thus includes the input winding 13 serially coupled to the building ground 8 by the capacitance Cgl provided by antenna 4 while the branch circuit 10 includes the input winding 14 serially coupled to the building ground 8 by the capacitance Cg2 provided by antenna 5. The pair of branch circuits 9 and 10 are connected in electrical parallel configuration at the juncture point 15.

A source of periodic voltage is provided by an electronic oscillator unit 16 which is electrically interconnected between the building or structure ground 8 and the juncture 15 between the input windings 13 and 14 with the output defining an electrical system ground as illustrated at 17. The oscillator 16 may consist of any one of a number of conventional electrical oscillating type circuits well known to those skilled in the art and may, for example, provide a 250 volt AC. power sup ply and further discussion thereof is deemed unnecessary.

A shunting circuit 18 is connected to a juncture point 19 located between antenna 4 and input winding 13 and to a juncture point 20 located between the antenna 5 and input winding 14 and is further connected to the building or structure ground 8. Specifically, a plurality of resistive elements 21, 22 and 23 are connected in series with resistor 21 connected to the juncture 19 and resistor 23 connected to the juncture 20. The resistive element or balance potentiometer 22 provides a selectively variable output tap 24 which is electrically coupled to the building ground 8 through a plurality of serially connected capacitive type elements 25 and 26. The trim capacitor 26 provides a selectively variable capacitance while the tap 24 of the balance potentiometer 22 may be selectively positioned to vary the shunting impedance from the antennae 4 and 5 to the building ground.

A selectively variable capacitor 27 is connected between juncture 15 and juncture 19 which is thus electrically connected in a parallel configuration to the input winding 13. The capacitor 27 could optionally be electrically connected in parallel with the input winding 14.

The transformers l1 and 12 are preferably coupled so that their windings are serially connected in electrical opposition. Thus, an output winding 28 of transformer 11 and an output winding 29 of transformer 12 are serially connected in electrical opposition at a juncture point 30 with the opposite ends of windings 28 and 29 providing outputs at points 31 and 32, respectively, svhich, in turn, provide an output to a control circuit The control circuit 33 includes an operational amplifier 34 having a first input35 connected to the output point 32 and a second input 36 coupled to the output point 31 through a serially connected resistor 37 and capacitor 38. The output point 31 is coupled to an electrical system ground as at 31a. The operational-amplifier 34 provides an alternating output signal at 39 which is fed back through a resistor 40 to the input 36.

A conversion circuit 41 is connected to receive the output of the operational amplifier 34 and includes a filtering capacitor 42 which is electrically connected to the output 39. A diode 43 includes an anode 44 which is electrically connected to the capacitor 42 and a cathode 45 which is electrically coupled to the system ground 46. A diode 47 includes a cathode 48 connected to the capacitor 42 and an anode 49 which is electrically coupled to the system ground 46 through a parallel connected capacitor 50 and potentiometer 51. The potentiometer 51 has a selectively variable output tap 52 which provides a constant output voltage having a magnitude varying in accordance with the varying magnitude of the alternating output signal received from the output 39. The conversion circuit 41 thus operates as an AC to DC converter while tap 52 can be selectively varied to manually adjust the sensitivity of the electrical'control circuit.

An electriealcomparator circuit 53 includes a comparator 54 having a first input 55 electrically coupled to the output tap 52 of the variable resistor 51. The input terminal 55 is also coupled to the system ground 46 through a Zener diode 56 which provides a maximum voltage limit control. lnput terminal 55 may also be selectively connected to the system ground 46 through a potentiometer 57 by the closure of a pair of contacts 58 which are controlled by the positioning of the elevator car door.

The comparator 54 further provides a second input 59 which is connected to a negative voltage source with respect to system ground 46 and designated as V through a pair of serially connected resistors 60 and 61. A juncture point 62 between resistors 60 and 61 is coupled to the system ground 46 through parallel connected potentiometer 63 and capacitor 64. The comparator 54 may selectively provide an output at 65 which is fed back to input 59through a resistor 66. A Zener diode 66a is connected across inputs 55 and 59 to provide a maximum voltage limit for the comparator 54. In operation, a preset voltage is established at input 59 which may bevaried by the selective variance of resistance provided by potentiometer 63. in order for the comparator to supply a positive outputat 65, the input voltage appearing at the first input 55 must rise to the voltage level appearing at the second input terminal 59.

ground 46. Whenever the voltage occurring at theoutput 65 of comparator 54 rises to a sufficiently positive level, the controlled rectifier 68 is rendered conductive thereby energizing the relay 69 which provides The comparator 54 may be prevented from responding to an output appearing at tap 52 when the elevator car door reaches a predetermined position. In a preferred embodiment, the contacts 58 close whenever the elevator car door approaches a substantially closed position thereby effectively connecting the input terminal to the system ground through resistor 57 and preventing the voltage occurring at input 55 from attaining the preset voltage level at input 59.

A switching circuit 67 is connected to the output of the comparator circuit 53 and includes a controlled rectifier 68 and a relay 69. Specifically, the controlled rectifier 68 provides a controlled gate 70 which is connected to the output 65 through a serially connected diode 71, resistor 72 and resistor 73. A juncture 74 between resistors 72 and 73 is coupled to the system ground 46 through a capacitor 75 while the input gate of the controlled rectifier68 is alsocoupled to the system ground 46 through a resistor 76. The controlled rectifier 68 may be selected from any one of a number of solid state switching devices and is embodied as a silicon controlled rectifier having an anode 77 which is electrically connected to a voltage source designated as V through the relay coil 69 and further provides a cathode 78 which is electrically coupled to the system contacts within the door control circuit for modifying the operation of the elevator car door. The elevator door operating means may thus be modified by the operation of the proximity detector to stop the closing motion of the elevator door when sensing the presence of an object within the closing path thereof and may optionally further operate to reopen the door to either a full or. partially opened position.

In summary, the output windings 28 and 29 of transformers l1 and 12, respectively, provide an electrical output in response to a change of impedence to ground of one antenna with respect to the other due to the proximity of load to such antenna. Due to the generally irregular shapes and sizes of loads handled or carried by elevator cars, the approach of a load to the leading edge of the car door effectively unbalances the antenna I to ground capacitance relationship between the two an tennae 4 and 5. For instance, should a passenger be in closer proximity to antenna 4 than to antenna 5, the capacitance Cgl of antenna 4 to the building ground will effectively increase to a greater extent than the capacitance Cg2 provided by antenna 5 thereby providing unequal voltages across the input windings 13 and 14 of the transformers 11 and 12, respectively. The opposing unequal output existing in output windings 28 and 29 thus provides an output to the operational amplifier 34 which supplies an alternating output signal. The conversion circuit 41 is connected to receive the alternating output signal provided by the operational amplifier 34 and provides a direct current (DC) output voltage having a magnitude varying in accordance with the varying magnitude of the alternating output signal. The variable tap 52 maybe adjusted to selectively vary the sensitivity of the output control circuit and provides an input signal to the comparator 54. Wh'enthe voltage to input 55 attains the preset voltage level supplied to input 59, the comparator 54 operates to rapidly switch its output voltage at output 65 to thereby fire or energize the output switching circuit to modify the door operating sequence. The adjustable preset voltage applied to the input 59 of comparator 54 thus establishes a selective trigger level which may be set according to desired operating conditions to provide variable responses. I

The balance potentiometer 22 of the shunting circuit 18 can beadjus-ted to vary the proportionate impedance shunting the antennae 4 and 5 to the building ground 8 to thereby balance the steady state of nonobstructed doorway condition thus providing an equal or balanced energization for the two transformers 11 and 12. The variable adjustment of the trim capacitor 26 advantageously controls the sensitivity of the antennae 4 and 5 according to the desired operating condition. The adjustment of the capacitor 27 which is parallel connected to one of the two input windings-13 or 14 further provides a balancing adjustment between the transformers 11 and 12 to compensate for slight electrical operating variances between the two transformers.

The proximity detector disclosed herein thus provides a highly desirable and economical safetysystem for use with elevator doors which operates with sensitivity and reliability to prevent injuries to passengers and other loads.

Various modes of carrying out the invention are contemplated as being within the scope of the following claims particularly pointing out and distinctly claiming the subject matter which is regarded as the invention.

1 claim:

1. [n a system providing a load transporting vehicle having a door, door-operating means connected for opening and closing said door, and a proximity detector connected to said door and to said door-operating means for selectively modifying the operation of said door, said detector comprising: a capacitance means including a sensing means secured to said door and responsive to the presence of load adjacent to the edge of said door; a transformer means having an input winding means including circuit means connected with said capacitance means to form a pair of parallel connected electrical paths for supplying first and second signals, each of said paths including a different portion of said capacitance means and a different portion of said transformer means; a source of periodic voltage electrically connected to said circuit means for energizing said parallel paths; said transformer means having an output winding means coupled to the different portions of said input winding means for establishing an output including a pair of oppositely polarized signals and varying according to the variance between said first and second signals in said pair of parallel paths, and a control circuit connected to said output winding means and to said door-operating means for selectively modifying the operation of said door in response to the sensed presence of load adjacent to the edge of said 7 door.

2. The system of claim I, wherein said detector includes a variable impedance means connected to said capacitance means and to said source and including a common adjustable output means to provide a pair of adjustable, inversely proportional impedance in said parallel paths and in parallel with the different portions of said capacitance means.

3. ln an elevator system, a structure having a hoistway provided with a landing and an elevator car disposed for movement in the hoistway from a position displaced from the landing to a predetermined position adjacent the landing to serve the landing for transporting load, said car having a door, door-operating means for opening and closing said door, a proximity detector connected to said door and to said door-operating means for selectively modifying the operation of said door in response to the sensed presence of load within the closing path of said door, said detector comprising:

first and second antennae mounted along the edge of said door to be responsive to the presence of load in proximity thereto,

a first input winding of a first transformer electrically connected in series to said first antenna and to an electrical ground by the capacitive coupling between said first antenna and said structure to form a first branch circuit,

a second input winding of a second transformer electrically connected in series to said second antenna and to an electrical ground by the capacitive coupling between said second antenna and said structure to form a second branch circuit, said first and second branch circuits electrically connected in parallel,

a source of periodic voltage connected to said first and second branch circuits, said first and second output transformers including first and second output windings, respectively, said output windings connected in series electrical opposition and providing an output in response to a change in impedance to ground of one antenna with respect to the other due to the proximity of load to such antenna, and a control circuit connected to said output and to said door-operating means for selectively modifying the operation of said door.

4. The system of claim 3, and includin a shunting circuit having a variable impedance connected to said electrical ground and to the junctures between said antennae and said input windings for providing a shunting impedance between each of said antennae and said electrical ground.

5. The system of claim 4, wherein said shunting circuit includes a resistive element electrically connected between said junctures and providing a selectively variable output tap electrically connected to said electrical ground.

6. The system of claim 4, wherein said shunting circuit includes a selectively variable capacitive element electrically connected between said junctures and said electrical ground.

7. The system of claim 3, and including a selectively variable capacitive element electrically connected in parallel to said first input winding.

8. The system of claim 3, wherein said control circuit includes an operational amplifier electrically connected to said output of said output transformers for providing an alternating output signal, a conversion circuit connected to said operational amplifier for providing a constant output voltage having a magnitude varying in accordance with the varying magnitude of said alternating output signal, an electrical comparator circuit having a first input electrically connected to said conversion circuit and a second input connected to a voltage source supplying a preset voltage, said comparator circuit providing an output signal when said output voltage from said conversion circuit supplied to said first input attains the preset voltage level supplied to said second input, and a switching circuit connected to receive said output signal from said comparator circuit for supplying a control signal to said door-operating means for modifying the opening and closing of said door.

9. The system of claim 8, wherein said control circuit further includes a de-activating circuit selectively electrically connected to said first input of said comparator in response to said door attaining a preselected position to prevent said switching circuit from modifying the operation of said door.

10. The system of claim 9, wherein said de-activating circuit includes means for preventing said output voltage supplied by said conversion circuit from attaining the preset voltage level supplied to said second input of said comparator circuit.

11. The system of claim 9, wherein said de-activating circuit further includes a pair of contacts controlled by selected voltage constituting the system electrical electrically connected to receive the output of said comparator, and a control relay electrically connected to the output of said controlled rectifier. 

1. In a system providing a load transporting vehicle having a door, door-operating means connected for opening and closing said door, and a proximity detector connected to said door and to said door-operating means for selectively modifying the operation of said door, said detector comprising: a capacitance means including a sensing means secured to said door and responsive to the presence of load adjacent to the edge of said door; a transformer means having an input winding means including circuit means connected with said capacitance means to form a pair of parallel connected electrical paths for supplying first and second signals, each of said paths including a different portion of said capacitance means and a different portion of said transformer means; a source of periodic voltage electrically connected to said circuit means for energizing said parallel paths; said transformer means having an output winding means coupled to the different portions of said input winding means for establishing an output including a pair of oppositely polarized signals and varying according to the variance between said first and second signals in said pair of parallel paths, and a control circuit connected to said output winding means and to said dooroperating means for selectively modifying the operation of said door in response to the sensed presence of load adjacent to the edge of said door.
 2. The system of claim 1, wherein said detector includes a variable impedance means connected to said capacitance means and to said source and including a common adjustable output means to provide a pair of adjustable, inversely proportional Impedance in said parallel paths and in parallel with the different portions of said capacitance means.
 3. In an elevator system, a structure having a hoistway provided with a landing and an elevator car disposed for movement in the hoistway from a position displaced from the landing to a predetermined position adjacent the landing to serve the landing for transporting load, said car having a door, door-operating means for opening and closing said door, a proximity detector connected to said door and to said door-operating means for selectively modifying the operation of said door in response to the sensed presence of load within the closing path of said door, said detector comprising: first and second antennae mounted along the edge of said door to be responsive to the presence of load in proximity thereto, a first input winding of a first transformer electrically connected in series to said first antenna and to an electrical ground by the capacitive coupling between said first antenna and said structure to form a first branch circuit, a second input winding of a second transformer electrically connected in series to said second antenna and to an electrical ground by the capacitive coupling between said second antenna and said structure to form a second branch circuit, said first and second branch circuits electrically connected in parallel, a source of periodic voltage connected to said first and second branch circuits, said first and second output transformers including first and second output windings, respectively, said output windings connected in series electrical opposition and providing an output in response to a change in impedance to ground of one antenna with respect to the other due to the proximity of load to such antenna, and a control circuit connected to said output and to said door-operating means for selectively modifying the operation of said door.
 4. The system of claim 3, and including a shunting circuit having a variable impedance connected to said electrical ground and to the junctures between said antennae and said input windings for providing a shunting impedance between each of said antennae and said electrical ground.
 5. The system of claim 4, wherein said shunting circuit includes a resistive element electrically connected between said junctures and providing a selectively variable output tap electrically connected to said electrical ground.
 6. The system of claim 4, wherein said shunting circuit includes a selectively variable capacitive element electrically connected between said junctures and said electrical ground.
 7. The system of claim 3, and including a selectively variable capacitive element electrically connected in parallel to said first input winding.
 8. The system of claim 3, wherein said control circuit includes an operational amplifier electrically connected to said output of said output transformers for providing an alternating output signal, a conversion circuit connected to said operational amplifier for providing a constant output voltage having a magnitude varying in accordance with the varying magnitude of said alternating output signal, an electrical comparator circuit having a first input electrically connected to said conversion circuit and a second input connected to a voltage source supplying a preset voltage, said comparator circuit providing an output signal when said output voltage from said conversion circuit supplied to said first input attains the preset voltage level supplied to said second input, and a switching circuit connected to receive said output signal from said comparator circuit for supplying a control signal to said door-operating means for modifying the opening and closing of said door.
 9. The system of claim 8, wherein said control circuit further includes a de-activating circuit selectively electrically connected to said first input of said comparator in response to said door attaining a preselected position to prevent said switching circuit from modifyIng the operation of said door.
 10. The system of claim 9, wherein said de-activating circuit includes means for preventing said output voltage supplied by said conversion circuit from attaining the preset voltage level supplied to said second input of said comparator circuit.
 11. The system of claim 9, wherein said de-activating circuit further includes a pair of contacts controlled by the positioning of said door for selectively electrically connecting said first input of said comparator to a preselected voltage constituting the system electrical ground.
 12. The system of claim 8, wherein said switching circuit includes a controlled rectifier having an input gate electrically connected to receive the output of said comparator, and a control relay electrically connected to the output of said controlled rectifier. 